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Stroke. 2000;31:1799-1801

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(Stroke. 2000;31:1799.)
© 2000 American Heart Association, Inc.

Editorial

Cerebrovascular Monitoring During Carotid Endarterectomy

Viken L. Babikian, MD Nancy L. Cantelmo, MD

From the Departments of Neurology (V.L.B.) and Surgery (N.L.C.), Boston University School of Medicine, Boston, Mass.

Correspondence to Viken Babikian, MD, Department of Neurology, Boston University School of Medicine, Boston VA Medical Center, 150 South Huntington Ave, Boston, MA 02130.


Key Words: carotid endarterectomy • complications • editorials

Carotid endarterectomy (CEA) is recognized today as the treatment of choice for selected patients with symptomatic or asymptomatic, severe, extracranial internal carotid artery stenosis. Introduced in the 1950s and perfected during the subsequent years, the number of CEAs has increased since the early 1990s1 to an annual rate exceeding 130 000 cases.2 CEA is the most frequently performed noncardiac vascular procedure in the United States.2

Stroke, myocardial infarction, and death are the major complications of CEA, myocardial infarction being a frequent cause of death.3 In 1995, an Ad Hoc Committee for the American Heart Association published guidelines which indicated that for the procedure to be efficacious, a surgeon’s combined morbidity and mortality rate should be <6% for symptomatic patients and <3% for those who are asymptomatic.4 Large CEA trials have shown that these figures can be achieved at selected centers.5 6 There is an increasing concern, however, that these rates may be exceeded in wider practice.7 In a recently published prospective study at a single medical center,8 a combined stroke and death rate of 8.3% was reported, and the rate was 11.1% when symptomatic patients were analyzed as a separate group. In addition, although the mortality rate for Medicare beneficiaries decreased from 1985 to 1996, it did not attain the rates of the trials,9 and it was 2.5% at hospitals with a low volume that did not participate in the CEA trials.7 More than likely, rates vary among surgeons and hospitals.7 10 11

Stroke is the most common major complication of CEA. Its incidence was 5.5% in the North American Symptomatic Carotid Endarterectomy Trial (NASCET)6 and 6.5% for patients with moderate stenosis in the European Carotid Surgery Trial.12 Although both embolism and hemodynamic compromise can lead to perioperative brain ischemia, embolism is increasingly considered the more common mechanism.13 Research studies of the past 20 years have shown that these mechanisms are not mutually exclusive, but that different phases of the operation are more susceptible to be affected by one or the other mechanism. Embolism occurs during the dissection phase of the surgery, at shunt insertion, at clamp release, and during the 12-hour period immediately after surgery.14 The latter may be responsible for >25% of all infarcts. Some degree of hemodynamic impairment occurs in most patients at cross-clamping. It is severe in approximately 10% to 20% of cases, probably because of inadequate collateral channels. These patients are considered candidates for shunting. In addition to the preceding "predictable" causes of brain ischemia, unexpected incidents, such as intraoperative internal carotid artery thrombosis,15 can occur and lead to distal ischemia, and in a small percentage of patients a hyperperfusion syndrome develops postoperatively, leading to brain edema and hemorrhage.

Thus, stroke occurring in the context of CEA can be the result of various mechanisms, some of which are unexpected. Not surprisingly, it has been difficult to predict and avoid. This is especially true because monitoring techniques used prior to transcranial Doppler ultrasonography were able to detect the embolic process only when evidence of brain dysfunction emerged. As a result, the impact of neuromonitoring during CEA has remained limited.

In an article published in this issue of Stroke, Ackerstaff et al16 report that intraoperative monitoring with transcranial Doppler can identify patients at an increased risk for perioperative, stroke. The authors found that 4 transcranial Doppler variables were independently associated with perioperative stroke. They consisted of embolism during the dissection and wound-closure phases of surgery, a drop of >=90% in the middle cerebral flow velocity at cross-clamping, and an increase of >=100% increase in the pulsatility index at clamp release.16 This substantial study is based on data pooled from 2 medical centers, and with 1058 patients is one of the largest published series based on transcranial Doppler technology. The 2 medical centers have had extensive experience with cerebrovascular monitoring, and a review of the methods section shows that the investigators have addressed technical issues carefully. The principal strength of the report is the identification of intraoperative transcranial Doppler variables that should alert the surgeon to take appropriate measures to avoid brain ischemia, possibly reducing the perioperative stroke rate. Because cerebrovascular monitoring can be implemented at other centers, these findings should be of interest to all surgeons who perform CEAs. The question raised is whether, based on the findings of this study, transcranial Doppler monitoring should be recommended for all CEAs.

The study of Ackerstaff et al16 has a number of limitations, of which 4 are briefly presented here. Foremost is the retrospective nature of the investigation. Strokes were detected by chart review, and a neurologist was consulted only "in patients with possible postoperative cerebral deficit." It is reasonable to assume that some strokes were missed.17 There were also differences between the 2 centers pertaining both to surgical technique (for example, when selecting candidates for shunting) and to transcranial Doppler technique or findings. As seen in the article’s Table 1, emboli were detected in Seattle more often than in Nieuwegein during all phases of the operation. Moreover, the usefulness of the study’s findings regarding embolism during the dissection and wound-closure phases of the operation remain limited because specific criteria regarding clinically relevant embolism, such as characteristics and frequency of emboli, were not presented. Because emboli are not infrequent during CEA, more specific criteria would be helpful to the surgeon when occasional signals are detected during the course of surgery. Finally, the study does not provide data about monitoring during the hours immediately after surgery,18 19 a period recognized to have an increased incidence of embolism. Although this is not a "fault" of the study, it limits the clinical applicability. Thus, in the era of evidence-based medicine established on prospective, double-blind, randomized investigations, this study does present some important limitations.

The study of Ackerstaff et al16 is by no means the first to evaluate the efficacy of transcranial Doppler monitoring during CEA. Previous studies by the same investigators20 21 and others19 22 have shown the usefulness of cerebrovascular monitoring when selecting candidates for shunting,22 and they have confirmed the ability of the technique to detect intraoperative embolism associated with clinically silent20 23 or evident19 21 brain infarction. On the basis of findings derived from monitoring, some investigators have initiated either intravenous dextran-4018 or platelet glycoprotein IIB/IIIA receptor antagonist24 therapy during the immediate postoperative period and have shown a subsequent decrease of the emboli count and a reduction of perioperative morbidity.18 The quality of the evidence in these well-designed, large, prospective studies18 19 can be rated class I or II when the American Academy of Neurology assessment criteria are used.25

When the findings of the study by Ackerstaff et al16 are taken in conjunction with those of the preceding investigations, it can be seen that they present reasonably strong evidence supporting a wider use of transcranial Doppler monitoring during CEA. Available data strongly suggest that a decrease of >85% of the ipsilateral middle cerebral artery flow velocity at cross-clamping and >25 emboli per 10-minute interval or >50 emboli per hour during the immediate postoperative period are associated with brain infarction.18 19 22 These criteria cannot be considered definitive at the present time, and the need for appropriately designed studies to establish definitive transcranial Doppler criteria and efficacy of monitoring is recognized. The proposed studies would need to enroll large numbers of patients to demonstrate a reduction in the rate of stroke, a relatively infrequent complication. Several factors, including cost, make it unlikely that such studies will be completed in the near future. The lack of "perfect data" should not lead to therapeutic paralysis, however. Until the time when such studies are completed, a wider use of transcranial Doppler monitoring seems reasonable because it can possibly improve the outcome of CEA.26

In a 1997 editorial in Stroke, Goldstein et al27 recognized the high complication rate of CEA and had a "call for action" recommending that the complication rate be audited by the Joint Commission on Accreditation of Healthcare Organizations as a condition of hospital certification. We concur with Goldstein et al. Recently published data show an improvement in the use and performance of CEA following the introduction of local surgical audits, and they suggest the complication rate can be reduced by locally organized quality control programs.28 29 There is also an increased awareness among physicians of the need to know the CEA morbidity and mortality data of the operating surgeon when the procedure is recommended to a patient.30 It is indeed a patient’s right to know the surgeon’s complication rate, as well as the results of large studies such as NASCET.

We support making public the CEA morbidity and mortality data of every medical center and, in addition, recommend the introduction of locally organized quality control programs. The aim of these multidisciplinary programs is to gather morbidity and mortality data and to devise ways to improve outcome. Careful local examination of surgical outcomes can identify specific areas of improvement. Examples of this can be seen in Leicester, UK, with CEA29 and in northern New England with coronary artery bypass graft surgery.31 National and local vascular surgery societies, the American Heart Association, and other professional associations could also do more to facilitate these programs by organizing sessions dedicated to outcome analysis at their annual meetings and by establishing templates of programs that could be implemented at the local level.

The introduction of cerebrovascular monitoring during CEA should be seen in the context of programs to improve the outcome of the operation. Not all surgeons will find the information provided by cerebrovascular monitoring useful, nor should all use it. For surgeons who operate with very low perioperative stroke rates, monitoring will not substantially affect outcome, and it will add to the procedure’s cost at a time when reimbursements have decreased. However, at centers with low volume for CEA, and when surgeons-in-training perform the surgery, cerebrovascular monitoring, with its present limitations, promises to provide useful information. Several practical issues will have to be resolved before the introduction of cerebrovascular monitoring, including cost and the availability of trained personnel in the operating room. However, transcranial Doppler testing is already available throughout the world, and the skills to perform monitoring can be acquired by technologists with appropriate training.

The outcome of carotid endarterectomy can and should improve. The introduction of locally directed quality control programs with the help of professional associations could be a first step toward this goal.

Footnotes

The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.

References

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